MHC class I-deficient tumor cell

The term “MHC class I-deficient tumor cells” does not denote a single, defined molecule or receptor but rather a **phenotypic class of cancer cells** that have lost or markedly downregulated surface expression of major histocompatibility complex class I (MHC class I; human HLA-A, HLA-B, HLA-C) and/or components of the MHC class I antigen-processing and presentation pathway.[1][2][3][5] Such deficiency can arise through structural genetic alterations (for example deletions or inactivating mutations in class I heavy chain genes, β2-microglobulin, or TAP genes) or through reversible transcriptional, epigenetic, and post-transcriptional mechanisms affecting HLA class I heavy chains, β2-microglobulin, and antigen-processing machinery components.[1][3] Because MHC class I molecules are required to present intracellular peptides to CD8+ cytotoxic T lymphocytes, MHC class I-deficient tumor cells become poorly visible or invisible to CD8+ T cells, enabling **immune evasion** and favoring tumor growth, metastatic progression, and reduced infiltration by tumor-infiltrating lymphocytes.[1][2][3][4][5] Loss or downregulation of MHC class I expression is frequent in many solid tumors, including colorectal, melanoma, breast, prostate, lung, and head and neck cancers, and is often associated with worse prognosis.[1][3][4] At the same time, low MHC class I expression can sensitize such tumor cells to **natural killer (NK) cell–mediated cytotoxicity** via the “missing-self” mechanism, although tumors may also develop strategies to evade NK cells (for example via immunosuppressive factors or modulation of ligands for NK receptors).[1][3][4][5] In immuno-oncology, MHC class I-deficient tumor cells are therefore considered a **biologically and clinically important phenotype** and an indirect therapeutic target: their presence influences response or resistance to T cell–based therapies and supports the rationale for strategies that restore MHC class I expression, bypass the requirement for MHC class I, or exploit NK-cell–based approaches.[1][3][4]

Other names

MHC class I-deficient tumor cellsMHC-I-deficient tumor cellsMHC class I-low tumor cellsMHC-I-low tumor cellsMHC class I-negative tumor cellsMHC-I-negative tumor cellsMHC class I–loss tumor cellsMHC class I–downregulated tumor cells